1. Field of the Invention
The present invention relates to a piezoelectric electroacoustic transducer such as a piezoelectric receiver, piezoelectric sounder, piezoelectric speaker, and piezoelectric buzzer, and more particularly, to a diaphragm of a piezoelectric electroacoustic transducer.
2. Description of the Related Art
A piezoelectric electroacoustic transducer has been widely used for a piezoelectric receiver, piezoelectric buzzer, or other suitable device. This piezoelectric electroacoustic transducer typically includes a unimorph type diaphragm which is constructed by adhering a circular metallic plate to one surface of a circular piezoelectric ceramic plate, wherein the outer peripheral portion of the diaphragm is supported in a circular case, and wherein an opening of the case is closed by a cover. However, since the unimorph type diaphragm obtains bending vibration by adhering a ceramic plate, having an outer diameter that expands and contracts, to a metallic plate which does not change in size in accordance with a voltage application thereto, the unimorph type diaphragm has a drawback that the displacement thereof, and thus, the sound pressure thereof is minimal.
Japanese Unexamined Patent Application Publication No. 61-205100, discloses a bimorph type diaphragm having a laminated structure including a plurality of piezoelectric ceramic layers. This diaphragm utilizes a sintered body obtained by laminating a plurality of ceramic green sheets and a plurality of electrodes, and then simultaneously firing them. These electrodes of the diaphragm are electrically interconnected via through holes provided at positions which do not restrain the vibration of the diaphragm. By constructing the bimorph diaphragm so that first and second vibrational regions thereof disposed in succession in the thickness direction vibrate in opposite directions, a larger displacement, and thus, a larger sound pressure than that of a unimorph diaphragm is achieved.
In the above-described bimorph diaphragm, however, in order to vibrate the diaphragm including, for example, three ceramic layers in a bending mode, it is necessary to interconnect one main surface electrode with one internal electrode via a through hole, to interconnect the other main surface electrode with the other internal electrode via a through hole, and further to apply an alternating voltage between each of the main surface electrodes and a corresponding internal electrode, as shown in FIG. 17 in the above-described publication. This requires a complicated interconnection between main surface electrodes and internal electrodes, and thus, increases the production cost of the bimorph diaphragm.
In addition, when the laminated body is being polarized, a voltage must be applied between an internal electrode, and top and bottom main surface electrodes. For example, where a diaphragm has a three-layered structure, as shown in FIG. 14 in the above-described publication, two through holes electrically connected to an internal electrode are connected to a connection electrode, and polarization is performed by applying a high voltage between the connection electrode and the top and bottom main surface electrodes. The conventional bimorph diaphragm, thus, has a drawback that it requires extending the internal electrode outside via through holes in order to perform polarization, which requires a complicated process such as the formation of the connection electrode.